The degrading effect of molecular contaminates in spacecraft has been well characterized. Molecules of hydrocarbon or plasticizer or other heavy molecular species are outgassed under vacuum conditions by polymeric components. Sources of these molecular contaminants can be paints, adhesives, or insulators. Such contaminants may also be outgassed from potting compounds, conformal coatings on printed circuit boards or the like and from other polymer compounds found inside electronic assemblies.
Such electronic assemblies are usually contained within an enclosure which is, typically, not hermetically sealed. The outgassing process may continue for years in the vacuum of space.
As an example, the contaminants outgassing from electronic components within a spacecraft will exit from the polymers into a surrounding electronic enclosure. The molecules will remain within the enclosure until they encounter an orifice whereby they exit into the inside volume of the spacecraft. The molecules will transfer from surface to surface within the volume of the spacecraft until they exit through a gap or orifice within the outer shell of the spacecraft. Some portion of these molecules may then impinge on a thermally sensitive exterior surface of the spacecraft, where some fraction of these molecules will adhere. Subsequent exposure to the sun and other ionizing radiation within space may cause a darkening of this molecular deposit, resulting in a gradual increase in temperature. Over a period of time, such areas develop into "hot spots" which may adversely affect the operation and/or the useable lifetime of the craft.
Spacecraft designers have only a limited means of controlling this outgassing process, such as by controlling the venting paths and directions of the molecules from the spacecraft. It is typically not practical or economical to totally reengineer the electronic enclosure assemblies themselves. However, this process of venting the molecules towards favorable directions does not completely solve the problem of contaminate deposits on the external surface of the spacecraft.
It is also known to treat the polymeric components themselves to substantially reduce the degree of outgassing. However, this approach is not a practical nor cost effective approach, and is only implemented in rare circumstances. In particular, it is not typically performed for electronic enclosures and assemblies, which are a major source of contaminates. For example, approximately fifteen percent of the total weight of a spacecraft may be made up of electronics-related polymeric materials. These materials are capable of outgassing contaminates into the spacecraft for years.
It is also not practical to hermetically seal the electronic enclosures, in that the resultant enclosure would need to be structurally reinforced in order to maintain a pressure of one atmosphere following liftoff of the spacecraft. The resultant "pressure vessel" structure would be substantially heavier than a nonhermetically sealed enclosure. This extra weight may result in a payload which is prohibitively expensive or technically impossible to place into orbit.